Liquid isoprene-butadiene rubber

ABSTRACT

The subject invention discloses a liquid isoprene-butadiene rubber (IBR) which is particularly valuable for use in making treads for high performance automobile tires, including race tires, that exhibit superior dry traction characteristics and durability. The isoprene-butadiene rubber of this invention is a liquid at room temperature and is comprised of repeat units which are derived from about 5 weight percent to about 95 weight percent isoprene and from about 5 weight percent to about 95 weight percent 1,3-butadiene, wherein the repeat units derived from isoprene and 1,3-butadiene are in essentially random order. The IBR of this invention also has a low number average molecular weight which is within the range of about 3,000 to about 50,000 and has a glass transition temperature which is within the range of about −50° C. to about 20° C. This invention more specifically discloses a tire tread compound which is comprised of (a) a rubbery elastomer, (b) about 20 phr to about 80 phr of an aromatic process oil, (c) about 35 phr to about 130 phr of a filler and (d) about 4 phr to about 40 phr of a liquid isoprene-butadiene rubber having a number average molecular weight which is within the range of about 3,000 to about 50,000, a glass transition temperature which is within the range of about −50° C. to about 20° C., and wherein the liquid isoprene-butadiene polymer exhibits only one glass transition temperature.

1. This is a continuation-in-part of U.S. patent application Ser. No.09/249,470, filed on Feb. 12, 1999 (now pending).

BACKGROUND OF THE INVENTION

2. It is important for high performance tires and race tires to haveexcellent traction characteristics. In fact, race drivers frequentlyattribute winning or losing automobile races to their tires. Forinstance, superior dry traction characteristics allow drivers to gofaster, giving them an edge over other drivers having tires that exhibitinferior traction characteristics. Durability can also be important inracing. For example, it is highly advantageous for a set of tires to bedurable enough to finish a race without being replaced which, of course,eliminates the need for changing tires during a pit stop. It is, ofcourse, always advantageous for high performance automobile tires tohave good durability for increased tire life.

3. Tire tread compounds for race tires and high performance tires areformulated to attain the best possible combination of tractioncharacteristics and durability. The traditional problem associated withthis approach is that traction characteristics and skid resistancecharacteristics are generally compromised to attain better durability.In order to balance these two inconsistent properties, mixtures ofvarious types of synthetic and natural rubber are normally utilized inhigh performance tire treads. For instance, various mixtures ofstyrene-butadiene rubber and polybutadiene rubber are commonly used as arubbery material for high performance automobile tire treads. Highlevels of aromatic processing oils are also normally included in highperformance tire tread formulations to increase dry tractioncharacteristics (see U.S. Pat. No. 4,861,131). However, high levels ofaromatic oils in tread compounds typically reduce the durability of thetire. Thus, the ability to improve the traction characteristics of atire tread without sacrificing durability is often elusive.

4. U.S. Pat. No. 4,843,120 discloses that tires having improvedperformance characteristics can be prepared by utilizing rubberypolymers having multiple glass transition temperatures as the treadrubber. These rubbery polymers having multiple glass transitiontemperatures exhibit a first glass transition temperature which iswithin the range of about −110°C. to −20°C. and exhibit a second glasstransition temperature which is within the range of about −50°C. to 0°C.According to U.S. Pat. No. 4,843,120, these polymers are made bypolymerizing at least one conjugated diolefin monomer in a firstreaction zone at a temperature and under conditions sufficient toproduce a first polymeric segment having a glass transition temperaturewhich is between −110°C. and 20°C. and subsequently continuing saidpolymerization in a second reaction zone at a temperature and underconditions sufficient to produce a second polymeric segment having aglass transition temperature which is between −20°C. and 20°C. Suchpolymerizations are normally catalyzed with an organolithium catalystand are normally carried out in an inert organic solvent.

5. U.S. Pat. No. 5,137,998 discloses a process for preparing a rubberyterpolymer of styrene, isoprene and butadiene having multiple glasstransition temperatures and having an excellent combination ofproperties for use in making tire treads which comprises:terpolymerizing styrene, isoprene and 1,3-butadiene in an organicsolvent at a temperature of no more than about 40°C. in the presence of(a) at least one member selected from the group consisting oftripiperidino phosphine oxide and alkali metal alkoxides and (b) anorganolithium compound.

6. U.S. Pat. No. 5,047,483 discloses a pneumatic tire having an outercircumferential tread where said tread is a sulfur-cured rubbercomposition comprised of, based on 100 parts by weight rubber (phr), (A)about 10 to about 90 parts by weight of a styrene, isoprene, butadieneterpolymer rubber (SIBR) and (B) about 70 to about 30 weight percent ofat least one of cis 1,4-polyisoprene rubber and cis 1,4-polybutadienerubber wherein said SIBR rubber is comprised of (1) about 10 to about 35weight percent bound styrene, (2) about 30 to about 50 weight percentbound isoprene and (3) about 30 to about 40 weight percent boundbutadiene and is characterized by having a single glass transitiontemperature (Tg) which is in the range of about −10°C. to about −40°C.and, further, the said bound butadiene structure contains about 30 toabout 40 percent 1,2-vinyl units, the said bound isoprene structurecontains about 10 to about 30 percent 3,4-units, and the sum of thepercent 1,2-vinyl units of the bound butadiene and the percent 3,4-unitsof the bound isoprene is in the range of about 40 to about 70 percent.

7. U.S. Pat. No. 5,272,220 discloses a styrene-isoprene-butadiene rubberwhich is particularly valuable for use in making truck tire treads whichexhibit improved rolling resistance and tread wear characteristics, saidrubber being comprised of repeat units which are derived from about 5weight percent to about 20 weight percent styrene, from about 7 weightpercent to about 35 weight percent isoprene and from about 55 weightpercent to about 88 weight percent 1,3-butadiene, wherein the repeatunits derived from styrene, isoprene and 1,3-butadiene are inessentially random order, wherein from about 25 percent to about 40percent of the repeat units derived from the 1,3-butadiene are of thecis-microstructure, wherein from about 40 percent to about 60 percent ofthe repeat units derived from the 1,3-butadiene are of thetrans-microstructure, wherein from about 5 percent to about 25 percentof the repeat units derived from the 1,3-butadiene are of thevinyl-microstructure, wherein from about 75 percent to about 90 percentof the repeat units derived from the isoprene are of the1,4-microstructure, wherein from about 10 percent to about 25 percent ofthe repeat units derived from the isoprene are of the3,4-microstructure, wherein the rubber has a glass transitiontemperature which is within the range of about −90°C. to about −70°C.,wherein the rubber has a number average molecular weight which is withinthe range of about 150,000 to about 400,000, wherein the rubber has aweight average molecular weight of about 300,000 to about 800,000 andwherein the rubber has an inhomogeneity which is within the range ofabout 0.5 to about 1.5.

8. U.S. Pat. No. 5,405,927 discloses a pneumatic truck tire having anouter circumferential tread wherein said tread is a sulfur-cured rubbercomposition comprised of, based on 100 parts by weight of rubber, (a)from about 25 to about 75 parts of an isoprene-butadiene rubber, saidrubber being comprised of repeat units which are derived from about 20weight percent to about 50 weight percent isoprene and from about 50weight percent to about 80 weight percent 1,3-butadiene, wherein therepeat units derived from isoprene and 1,3-butadiene are in essentiallyrandom order, wherein from about 3 percent to about 10 percent of therepeat units in said rubber are 1,2-polybutadiene units, wherein fromabout 50 percent to about 70 percent of the repeat units in said rubberare 1,4-polybutadiene units, wherein from about 1 percent to about 4percent of the repeat units in said rubber are 3,4-polyisoprene units,wherein from about 25 percent to about 40 percent of the repeat units inthe polymer are 1,4-polyisoprene units, wherein the rubber has a glasstransition temperature which is within the range of about −90°C. toabout −75°C. and wherein the rubber has a Mooney viscosity which iswithin the range of about 55 to about 140; and (b) from about 25 toabout 75 parts of natural rubber.

SUMMARY OF THE INVENTION

9. The traction characteristics of high performance tires and race tirescan be improved without compromising durability by utilizing thetechnique of this invention. This approach involves substituting about 4phr (parts per hundred parts by weight of rubber) to about 40 phr ofliquid isoprene-butadiene rubber for a portion of the aromatic oilincluded in the tread rubber formulation. The liquid isoprene-butadienerubber employed in the tread rubber formulations of this inventiontypically has a number average molecular weight which is within therange of about 3,000 to about 50,000 and a glass transition temperaturewhich is within the range of about −50°C. to about 20°C. The tire treadrubber compounds of this invention will also normally contain about 20phr to about 80 phr of an aromatic process oil and about 35 phr to about130 phr of a filler.

10. This invention more specifically discloses a liquidisoprene-butadiene polymer which is comprised of repeat units which arederived from about 5 weight percent to about 95 weight percent isopreneand from about 5 weight percent to about 95 weight percent1,3-butadiene, wherein the repeat units derived from isoprene and1,3-butadiene are in essentially random order, wherein the liquidisoprene-butadiene polymer has a low number average molecular weightwhich is within the range of about 3,000 to about 50,000 and wherein theliquid isoprene-butadiene polymer has a glass transition temperaturewhich is within the range of about −50°C. to about 20°C.

11. The subject invention further reveals a tire tread compound which iscomprised of (a) a rubbery elastomer, (b) about 20 phr to about 80 phrof an aromatic process oil, (c) about 35 phr to about 130 phr of afiller and (d) about 4 phr to about 40 phr of a liquidisoprene-butadiene rubber having a number average molecular weight whichis within the range of about 3,000 to about 50,000 and a glasstransition temperature which is within the range of about −50°C. toabout 20°C.

12. The subject invention further reveals a tire which is comprised of agenerally toroidal-shaped carcass with an outer circumferential tread,two spaced beads, at least one ply extending from bead to bead andsidewalls extending radially from and connecting said tread to saidbeads; wherein said tread is adapted to be ground-contacting; whereinthe tread is comprised of (a) a rubbery elastomer, (b) about 20 phr toabout 80 phr of an aromatic process oil, (c) about 35 phr to about 130phr of a filler and (d) about 4 phr to about 40 phr of a liquidisoprene-butadiene rubber having a number average molecular weight whichis within the range of about 3,000 to about 50,000 and a glasstransition temperature which is within the range of about −50°C. toabout 20°C.

DETAILED DESCRIPTION OF THE INVENTION

13. The IBR of this invention is synthesized by solution polymerization.Such solution polymerizations will normally be carried out in ahydrocarbon solvent which can be one or more aromatic, paraffinic orcycloparaffinic compounds. These solvents will normally contain from 4to 10 carbon atoms per molecule and will be liquids under the conditionsof the polymerization. Some representative examples of suitable organicsolvents include pentane, isooctane, cyclohexane, normal hexane,benzene, toluene, xylene, ethylbenzene, and the like, alone or inadmixture.

14. In the solution polymerizations of this invention, there willnormally be from about 5 to about 35 weight percent monomers in thepolymerization medium. Such polymerization media are, of course,comprised of the organic solvent, 1,3-butadiene monomer and isoprenemonomer. In most cases, it will be preferred for the polymerizationmedium to contain from 10 to 30 weight percent monomers. It is generallymore preferred for the polymerization medium to contain 20 to 25 weightpercent monomer.

15. The monomer charge compositions utilized in the polymerizations ofthis invention will typically contain from about 5 weight percent toabout 95 weight percent isoprene and from about 5 weight percent toabout 95 weight percent 1,3-butadiene monomer. It is more typical forthe monomer charge composition to contain from about 20 weight percentto about 80 weight percent isoprene and from about 20 weight percent toabout 80 weight percent 1,3-butadiene. In most cases, the monomer chargecomposition will contain from about 30 weight percent to about 70 weightpercent isoprene and from about 30 weight percent to about 70 weightpercent 1,3-butadiene.

16. Since the copolymerization of the 1,3-butadiene monomer and isoprenemonomer is normally carried out to a high conversion, the ratio ofrepeat units in the liquid isoprene-butadiene polymer that are derivedfrom isoprene and 1,3-butadiene will be about the same as was employedin the monomer charge composition. Thus, the liquid isoprene-butadienepolymer will normally contain from about 5 weight percent to about 95weight percent bound isoprene and from about 5 weight percent to about95 weight percent bound 1,3-butadiene monomer. The liquidisoprene-butadiene polymer will typically contain from about 20 weightpercent to about 80 weight percent bound isoprene and from about 20weight percent to about 80 weight percent bound 1,3-butadiene monomer.In most cases, the liquid isoprene-butadiene polymer will contain fromabout 30 weight percent to about 70 weight percent bound isoprene andfrom about 30 weight percent to about 70 weight percent bound1,3-butadiene monomer.

17. The liquid isoprene-butadiene polymer of this invention can be madeby a batch process or continuously. It will normally be advantageous tosynthesize the isoprene-butadiene polymer on a continuous basis. In sucha continuous process, the monomers and an organolithium initiator arecontinuously fed into a reaction vessel or series of reaction vessels.The pressure in the reaction vessel is typically sufficient to maintaina substantially liquid phase under the conditions of the polymerizationreaction. The reaction medium will generally be maintained at atemperature which is within the range of about 20°C. to about 140°C.throughout the copolymerization. The reaction temperature willpreferably be within the range of about 40°C. to about 100°C. during thecourse of the copolymerization. It is generally most preferred for thereaction medium to be maintained at a temperature which is within therange of about 60°C. to 80°C. throughout the copolymerization.

18. The organolithium compounds which can be utilized as initiators inthe copolymerizations of this invention include organomonolithiumcompounds and organo multifunctional lithium compounds. The organomultifunctional lithium compounds will typically be organodilithiumcompounds or organotrilithium compounds. Some representative examples ofsuitable multifunctional organolithium compounds include1,4-dilithiobutane, 1,10-dilithiodecane, 1,20-dilithioeicosane,1,4-dilithiobenzene, 1,4-dilithionaphthalene, 9,10-dilithioanthracene,1,2-dilithio-1,2-diphenylethane, 1,3,5-trilithiopentane,1,5,15-trilithioeicosane, 1,3,5-trilithiocyclohexane,1,3,5,8-tetralithiodecane, 1,5,10,20-tetralithioeicosane,1,2,4,6-tetralithiocyclohexane, 4,4N-dilithiobiphenyl, and the like.

19. The organolithium compounds which can be utilized are normallyorganomonolithium compounds. The organolithium compounds which arepreferred can be represented by the formula R-Li, wherein R represents ahydrocarbyl radical containing from 1 to about 20 carbon atoms.Generally, such monofunctional organolithium compounds will contain from1 to about 10 carbon atoms. Some representative examples oforganolithium compounds which can be employed include methyllithium,ethyllithium, isopropyllithium, n-butyllithium, sec-butyllithium,noctyllithium, tert-octyllithium, n-decyllithium, phenyllithium,1-napthyllithium, 4-butylphenyllithium, p-tolyllithium,1-naphthyllithium, 4-butylphenyllithium, p-tolyllithium,4-phenylbutyllithium, cyclohexyllithium, 4-butylcyclohexyllithium and4-cyclohexylbutyllithium. The organolithium initiator will typically bean alkyl lithium compound; such as, n-butyl lithium.

20. The amount of organolithium initiator employed will be dependentupon the molecular weight which is desired for the liquidisoprene-butadiene polymer being synthesized. As a general rule in allanionic polymerizations, the molecular weight of the polymer produced isinversely proportional to the amount of initiator utilized. Since liquidisoprene-butadiene polymer having a relatively low molecular weight isbeing synthesized, the amount of initiator employed will be relativelylarge. As a general rule, from about 0.1 to about 2 phm (parts perhundred parts of monomer by weight) of the organolithium compound willbe employed. In most cases, it will be preferred to utilize from about0.2 to about 1 phm of the organolithium compound with it being mostpreferred to utilize from about 0.4 phm to 0.6 phm of the organolithiumcompound. In any case, an amount of organolithium initiator will beselected to result in the production of liquid isoprene-butadienepolymer having a number average molecular weight which is within therange of about 3,000 to about 50,000. The amount of organolithiuminitiator will preferably be selected to result in the production ofliquid isoprene-butadiene polymer having a number average molecularweight which is within the range of about 5,000 to about 30,000. Theamount of organolithium initiator will most preferably be selected toresult in the production of liquid isoprene-butadiene polymer having anumber average molecular weight which is within the range of about 8,000to about 18,000.

21. It is critical to carry out the copolymerization of 1,3-butadieneand styrene in the presence of a polar modifier, such asN,N,N′,N′-tetramethylethylenediamine (TMEDA), to attain a high glasstransition temperature which is within the range of about −50°C. to20°C. It is preferred for the isoprene-butadiene polymer to have a glasstransition temperature which is within the range of about −40°C. toabout 10°C. It is generally most preferred for the liquidisoprene-butadiene rubber to have a glass transition temperature whichis within the range of about −30°C. to about 0°C. Normally, a molarratio of the polar modified to lithium initiator of at least about 0.5:1will be utilized to attain the desired glass transition temperature. Inmost cases, a molar ratio of the polar modified to lithium initiatorwhich is within the range of about 0.5:1 to about 20:1 will be utilized.It is normally preferred to use a molar ratio of the polar modified tolithium initiator which is within the range of about 1:1 to about 10:1.It is typically preferred to employ a molar ratio of the polar modifiedto lithium initiator which is within the range of about 1.4:1 to about4:1. It is normally more preferred to employ a molar ratio of the polarmodified to lithium initiator which is within the range of about 1.6:1to about 2:1.

22. Ethers and tertiary amines which act as Lewis bases arerepresentative examples of polar modifiers that can be utilized. Somespecific examples of typical polar modifiers include diethyl ether,di-n-propyl ether, diisopropyl ether, di-n-butyl ether, tetrahydrofuran,dioxane, ethylene glycol dimethyl ether, ethylene glycol diethyl ether,diethylene glycol dimethyl ether, diethylene glycol diethyl ether,triethylene glycol dimethyl ether, trimethylamine, triethylamine,N,N,N′,N′-tetramethylethylenediamine, N-methyl morpholine, N-ethylmorpholine, N-phenyl morpholine and the like. Dipiperidinoethane,dipyrrolidinoethane, tetramethylethylene diamine, diethylene glycol,dimethyl ether, TMEDA and tetrahydrofuran are representative of highlypreferred modifiers: U.S. Pat. No. 4,022,959 describes the use of ethersand tertiary amines as polar modifiers in greater detail.

23. The isoprene-butadiene polymer produced by the copolymerization isrecovered from the organic solvent after the desired monomer conversionhas been attained. The isoprene-butadiene polymer can be recovered fromthe organic solvent by standard techniques. It is normally desirable toprecipitate the isoprene-butadiene polymer from the organic solvent bythe addition of lower alcohols containing from 1 to about 4 carbon atomsto the polymer solution. Suitable lower alcohols for precipitation ofthe isoprene-butadiene rubber from the polymer cement include methanol,ethanol, isopropyl alcohol, n-propyl alcohol and t-butyl alcohol. Theutilization of lower alcohols to precipitate the liquidisoprene-butadiene rubber from the polymer cement also “kills” theliving polymer chains by inactivating lithium end groups. After theisoprene-butadiene polymer is recovered from the organic solvent,steam-stripping can be employed to reduce the level of volatile organiccompounds in the polymer.

24. The repeat units derived from isoprene and 1,3-butadiene that are inthe liquid isoprene-butadiene rubber are in an essentially random order.The term “random” as used herein means that the repeat units which arederived from isoprene are well dispersed throughout the polymer and aremixed in with repeat units which are derived from 1,3-butadiene. Forpurposes of this patent, “random” means that over 60 percent of theisoprene in the IBR is present in blocks of three or less repeat units.

25. The liquid isoprene-butadiene rubber will exhibit only one glasstransition temperature that will be within the range of about −50°C. to20°C. It is preferred for the isoprene-butadiene polymer to have itsglass transition temperature at a temperature that is within the rangeof about −40°C. to about 10°C. It is generally most preferred for theliquid isoprene-butadiene rubber to have its glass transitiontemperature which is within the range of about −30°C. to about 0°C. Inany case, the liquid isoprene-butadiene polymer of this invention doesnot exhibit multiple glass transition temperatures. The liquidisoprene-butadiene rubber has a single glass transition temperature thatoccurs within the range of about −50°C. to 20°C.

26. For purposes of this patent application, polymer microstructures aredetermined by nuclear magnetic resonance spectrometry (NMR). Glasstransition temperatures are determined by differential scanningcalorimetry at a heating rate of 10°C. per minute and molecular weightsare determined by gel permeation chromatography (GPC).

27. The liquid isoprene-butadiene rubber of this invention isparticularly valuable for use in making treads for high performanceautomobile tires. Such high performance tires will normally include agenerally toroidal-shaped carcass with an outer circumferential tread,two spaced beads, at least one ply extending from bead to bead andsidewalls extending radially from and connecting said tread to saidbeads. The tread of such tires is, of course, adapted to beground-contacting. The treads of this invention are comprised of (a) arubbery elastomer, (b) about 20 phr (parts by weight per 100 parts byweight of the rubbery elastomer) to about 80 phr of an aromatic processoil, (c) about 35 phr to about 130 phr of a filler and (d) about 4 phrto about 40 phr of a liquid isoprene-butadiene polymer of thisinvention. Such treads provide tires with significantly improved drytraction without sacrificing durability.

28. In the tire tread formulations of this invention, the liquidisoprene-butadiene polymer is essentially substituted for a portion ofthe aromatic processing oil normally included in the tread compound. Theliquid isoprene-butadiene polymer will be added to the tread formulationin an amount that is within the range of about 4 phr to about 40 phr. Inmost cases, it is preferred for the liquid isoprene-butadiene polymer tobe present in the tread compound in an amount which is within the rangeof about 5 phr to about 25 phr. The amount of aromatic process oil addedto the tread compound will typically be within the range of about 20 phrto about 80 phr. In high performance passenger tire applications, theoil will typically be added in an amount that is within the range ofabout 20 phr to about 50 phr. In high performance passenger tireapplications, the oil will more typically be added in an amount that iswithin the range of about 30 phr to about 45 phr. In race tireapplications, the oil will typically be added in an amount that iswithin the range of about 50 phr to about 80 phr. In race tireapplications, the oil will more typically be added in an amount that iswithin the range of about 55 phr to about 70 phr.

29. The rubbery elastomer employed in the tread compound can be anyrubber or blend of rubbers that can be used in manufacturing highperformance tire treads. For instance, the rubbery elastomer can benatural rubber, emulsion styrene-butadiene rubber, solutionstyrene-butadiene rubber, styrene-isoprene-butadiene rubber or a mixturethereof. Such blends can also contain 3,4-polyisoprene,trans-1,4-polybutadiene or cis-1,4-polybutadiene rubber. In cases where3,4-polyisoprene, trans-1,4-polybutadiene or cis-1,4-polybutadienerubber are included in the tread compound, they will generally beemployed at a level of less than about 40 phr. For instance, the3,4-polyisoprene, trans-1,4-polybutadiene or the cis-1,4-polybutadienecan be employed in an amount which is within the range of about 5 phr toabout 35 phr. The 3,4-polyisoprene which can be utilized will normallyhave a 3,4-microstructure content of 55 percent to 80 percent asdetermined by NMR spectroscopy. The 3,4-polyisoprene will, accordingly,have a cis-1,4-microstructure content which is within the range of 20percent to 45 percent. The 3,4-polyisoprene will also normally have aglass transition temperature from −25°C. to 10°C. as determined bydifferential scanning calorimetry at a heating rate of 10°C./minute. Thecis-1,4-polybutadiene that can be employed in the tread compound willtypically have a cis-1,4-microstructure content of at least about 96percent as determined by NMR spectroscopy.

30. In many cases, it is preferred for the rubbery elastomer in thetread compound to be styrene-butadiene rubber. For instance, the treadcompound can be made using only styrene-butadiene rubber as the rubberyelastomer. However, the rubbery elastomer can also be a blend ofstyrene-butadiene rubber with natural rubber, 3,4-polyisoprene,cis-1,4-polybutadiene, trans-1,4-polybutadiene orstyrene-isoprene-butadiene rubber. In cases where the styrene-butadienerubber is blended with another rubbery polymer, the other rubberypolymer will normally be added in an amount that is within the range ofabout 5 phr to about 40 phr.

31. The tread compound will also contain at least one filler, such ascarbon black and/or silica. Clays and/or talc can be included in thefiller to reduce cost. The filler will normally be present in an amountwhich is within the range of about 35 phr to about 130 phr. In highperformance passenger tire applications, the filler will normally bepresent in an amount which is within the range of about 35 phr to about70 phr. In race tire applications, the filler will normally be presentin an amount that is within the range of about 70 phr to about 130 phr.The tread compound can also contain one or more resins; such as,coumarone-indene resin. The resin will normally be added in an amountthat is within the range of about 5 phr to about 60 phr in race tireapplications. In passenger tire applications, the resin will typicallybe added in an amount that is within the range of 0 phr to about 20 phr.In passenger tire applications, it is typically preferred for the treadcompound not to contain any resin.

32. The liquid isoprene-butadiene polymer containing blends of thisinvention can be compounded utilizing conventional ingredients andstandard techniques. For instance, the liquid isoprene-butadiene polymercontaining rubber compound will typically also include sulfur,accelerators, waxes, scorch inhibiting agents and processing aids. Inmost cases, the tread rubber formulation will be compounded with sulfurand/or a sulfur containing compound, at least one accelerator, at leastone antidegradant, at least one processing oil, zinc oxide, optionally atackifier resin, optionally a reinforcing resin, optionally one or morefatty acids, optionally a peptizer and optionally one or more scorchinhibiting agents. Such blends will normally contain from about 0.5 to 5phr (parts per hundred parts of rubber by weight) of sulfur and/or asulfur containing compound with 1 phr to 2.5 phr being preferred. It maybe desirable to utilize insoluble sulfur in cases where bloom is aproblem.

33. The blend will also normally include from 0.1 to 2.5 phr of at leastone accelerator with 0.2 to 1.5 phr being preferred. Antidegradants,such as antioxidants and antiozonants, will generally be included in theblend in amounts ranging from 0.25 to 10 phr with amounts in the rangeof 1 to 5 phr being preferred. The liquid isoprene-butadiene polymercontaining blends of this invention will also normally contain from 0.5to 10 phr of zinc oxide with 1 to 5 phr being preferred. These blendscan optionally contain from 0 to 30 phr of tackifier resins, 0 to 10 phrof reinforcing resins, 1 to 10 phr of fatty acids, 0 to 2.5 phr ofpeptizers and 0 to 1 phr of scorch inhibiting agents.

34. The liquid isoprene-butadiene rubber containing tread rubbercompounds of this invention can be used in tire treads in conjunctionwith ordinary tire manufacturing techniques. In other words, tires canbe built utilizing standard procedures with the liquidisoprene-butadiene rubber simply being substituted for a portion of thearomatic oil normally included in the tread rubber compound. After thetire has been built, it can be vulcanized using a normal tire curecycle. Tires made in accordance with this invention can be cured over awide temperature range. However, it is generally preferred for the tiresof this invention to be cured at a temperature ranging from about 132°C.(270°F.) to about 166°C. (330°F.).

35. This invention is illustrated by the following examples which aremerely for the purpose of illustration and are not to be regarded aslimiting the scope of the invention or the manner in which it can bepracticed. Unless specifically indicated otherwise, all parts andpercentages are given by weight.

EXAMPLE 1

36. In this experiment, 2240 g of a silica/alumina/molecular sieve driedpremix containing 19.7 weight percent of a 1,3-butadiene/isoprenemixture in hexanes was charged into a one-gallon (3.8 liter) reactor.The ratio of 1,3-butadiene to isoprene was 70:30. After the impuritylevel was determined to be 2 ppm, 8.4 ml of neatN,N,N′,N′-tetramethylethylene diamine (TMEDA) and 35 ml of a 1.6 Msolution of n-butyllithium (n-BuLi) in hexanes were added to thereactor. The molar ratio of TMEDA to n-BuLi was 1:1 and the targetnumber averaged molecular weight was 8,000.

37. The polymerization was carried out at 65°C. for 15 minutes. The GCanalysis of the residual monomers contained in the polymerizationmixture indicated that the polymerization was completed. Then, 3.5 ml ofneat ethanol was added to shortstop the polymerization and polymer wasremoved from the reactor and stabilized with 1 phm of antioxidant. Afterevaporating the hexanes solvent, the resulting polymer was dried in avacuum oven at 50°C.

38. The isoprene-butadiene copolymer produced was determined to have aglass transition temperature (Tg) at −29°C. It was also determined tohave a microstructure which contained 45 percent 1,2-polybutadieneunits, 19 percent 1,4-polybutadiene units, 9 percent 1,2-polyisopreneunits, 7 percent 1,4-polyisoprene units and 20 percent 3,4-polyisopreneunits.

EXAMPLES 2-5

39. The procedure described in Example 1 was utilized in these examplesexcept that the 1,3-butadiene to isoprene ratio and TMEDA/n-BuLi ratiowas varied. The butadiene/isoprene ratio (Ed/I) and TMEDA/n-butyllithium ratio employed in each of the examples are shown in Table I. Theglass transition temperature of each of the polymers produced is alsoshown in Table I. TABLE I Example Bd/I TMEDA/n-BuLi Tg 1 70/30   1/1−29° C. 2 70/30 0.5/1 −39° C. 3 50/50 0.75/1  −24° C. 4 30/70 0.5/1 −32°C. 5 30/70 0.7/1 −15° C.

40. The microstructure of the liquid isoprene-butadiene rubbers made areshown in Table II. TABLE II Example 1,2-PBd 1,4-PBd 1,2-PI 1,4-PI 3,4-PI1 45% 19% 9 7% 20% 2 42% 23% 5% 12% 18% 3 33% 14% 9% 12% 32% 4 19% 10%5% 29% 37% 5 20% 8% 11% 16% 45%

EXAMPLE 6

41. In this experiment, liquid isoprene-butadiene rubber having a numberaverage molecular weight of about 16,000 was synthesized in a reactorhaving a capacity of 500 gallons (1893 liters). In the procedure used,48 pounds (17.9 kg) of isoprene and 112 pounds (41.8 kg) of1,3-butadiene were mixed with 1840 pounds (686.7 kg) of polymerizationgrade hexanes in a premix tank. The monomer premix solution made had amonomer concentration of 8 percent. The monomer premix solution at atemperature of 70°F. (21°C.) was then passed over a silica gel bed at aliquid hour space velocity (LHSV) of about 1 hour to removetertiary-butyl catechol inhibitor, water and any other polar materialsthat might have been present. The monomer solution was then charged intothe polymerization reactor and heated to a temperature of 100°F.(38°C.). After the monomer solution had been charged into the reactor,2.478 pounds (925 g) of TMEDA was introduced into the reactor. Thecontents of the reactor were then agitated for approximately 5 minutesprior to the addition of 0.6 8301 pounds (254.8 g) of n-butyl lithiuminitiator. The level of n-butyl lithium initiator was about 0.43 pounds(160.5 g) per 100 pounds (37.3 kg) of monomer. The polymerizationreaction was completed in about 5 minutes. However, the polymerizationmixture in the reactor was agitated for an additional 25 minutes.

42. An antioxidant solution was made by charging 3.97 pounds (1.5 kg) ofa 30 percent Wingstay® K antioxidant in a hexane solution into adesolventizer tank. The antioxidant solution was made by mixing 1.19pounds (444.1 g) of the Wingstay® K antioxidant into 2.78 pounds (1037g) of hexane at a temperature of 70°F. (21°C.). A 10 percent shortstopsolution was also made by mixing 1.174 pounds (438.1 g) of rosin acidinto 10.223 pounds (3.8 kg) of hexane at a temperature of 70°(21°C.).The shortstop solution was charged into the desolventizer tank with carebeing taken to keep the desolventizer tank free of oxygen. It should benoted that rosin acid is generally not regarded as being an extremelyeffective shortstop. However, dissolved water was present in the streamand the water was probably responsible for much of the termination. Inany case, rosin acid does impart favorable properties in end useapplications and this was the primary reason that it was used.

43. The live polymer cement was charged into the antioxidant/shortstopheel in the desolventizer tank. Once transfer was complete, the cementwas slowly heated by a jacket glycol loop to establish a strong flow ofsolvent from an overhead condenser. Desolventization was continued withagitation until the concentrated polymer solution was below the agitatorblade. At this point, a nitrogen sparge was introduced to the polymercement to strip off remaining solvent and to assist in removal of theTMEDA to avoid a strong amine odor in the product. When the solidsanalysis reached about 99 percent, the polymer heel was dropped hot intodrums. The temperature at the end of the stripping step was about 210°F.(99°C.). It should be noted that the polymer became rather viscous anddifficult to remove from the vessel if allowed to cool much below 210°F.(99°C.).

EXAMPLES 7-10

44. In this series of experiments, the general procedure described inExample 6 was repeated with the level of n-butyl lithium initiator andTMEDA being adjusted to attain lower molecular weights. The initiatorlevel, TMEDA level and the number average molecular weight (Mn) of theisoprene-butadiene rubbers made is reported in Table III. TABLE IIIExample Mn n-BuLi TMEDA 6 16,000 0.43 phm 1.55 phm 7 14,000 0.46 phm1.66 phm 8 12,000 0.53 phm 1.93 phm 9 10,000 0.64 phm 2.32 phm 10 8,0000.80 phm 2.90 phm

EXAMPLES 11-14

45. In this series of experiments, tire tread compounds were made usingthe liquid isoprene-butadiene rubber of this invention. These tire treadcompounds were made by mixing 100 phr of styrene-butadiene rubber, 84.48phr of carbon black, 29.6 phr of aromatic process oil, 14.4 phr of aliquid isoprene-butadiene rubber having the butadiene/isoprene ratioshown in Table IV, 20.64 phr medium hard coumarone indene resin having amelting point of 100°C., 0.96 phr of stearic acid, 1.2 phr of zinc oxideand 0.67 phr of polymerized 1,2-dihydro-2,2,4-trimethylquinoline forabout 2.5 minutes to produce a non-productive compound. Thestyrene-butadiene rubber employed in these compounds had a glasstransition temperature of −16°C., a Mooney ML 1+4 viscosity of about 90,a bound styrene content of 32 percent and a vinyl content of 42 percent.The non-productive compound was then further mixed with 2.4 phr ofN-cyclohexyl benzothiazole-2-sulfenamide, 0.206 phr oftetramethylthiuram disulfide and 1.23 phr of rubber makers sulfur in aproductive step. The tread rubber compound made in Example 14 was acontrol and did not contain any liquid isoprene-butadiene rubber. Sincethe tread rubber compound of Example 14 did not contain the liquidisoprene-butadiene rubber, the level of aromatic process oil wasincreased to 44 phr.

46. The cured rubber samples were subsequently evaluated to determinephysical characteristics. These physical properties are reported inTable IV. TABLE IV Example 11 12 13 14 Butadiene/Isoprene 30/70 50/5070/30 — 300% Modulus, MPa 4.5 4.6 4.2 4.9 Break Strength, MPa 11.4 11.310.3 10.5 Elongation (%) 643 634 633 579 Energy 127 117 115 110 Hardness@ RT 75.5 76.4 75.2 74.5 Hardness @ 100° C. 44.4 44.5 44.5 45 Rebound @RT (%) 9.3 9.3 9.5 8.2 Rebound @ 100° C. (%) 26.1 27.2 26.9 30.5 DINAbrasion 244 265 265 282 G′ 50%, kPa* 427 410 413 438 J″ 50%, l/kPa*0.54 0.59 0.57 0.5

47. Typically high performance tires contain high levels of oil toincrease dry traction. Generally increasing the oil level increases drytraction (as is exemplified by higher J″ values) but reduces thedurability (300% Modulus, break strength and energy to break). As can beseen from Table IV, the use of the liquid isoprene-butadiene rubber ofthis invention in tire tread compounds improves dry traction whilemaintaining durability. By replacing 14.4 phr of oil with 14.4 phr ofthe liquid isoprene-butadiene rubbers, the dry traction (J″) of thecompounds are increased by 8-18 percent. The lab indicator for treadwear(DIN abrasion) shows improvement for all of the liquidisoprene-butadiene polymer containing blends. Thus, durability of allthe compounds containing the liquid isoprene-butadiene rubber is equalor improved.

48. While certain representative embodiments and details have been shownfor the purpose of illustrating the subject invention, it will beapparent to those skilled in this art that various changes andmodifications can be made therein without departing from the scope ofthe subject invention.

What is claimed is:
 1. A liquid isoprene-butadiene polymer which iscomprised of repeat units which are derived from about 5 weight percentto about 95 weight percent isoprene and from about 5 weight percent toabout 95 weight percent 1,3-butadiene, wherein the repeat units derivedfrom isoprene and 1,3-butadiene are in essentially random order, whereinthe liquid isoprene-butadiene polymer has a low number average molecularweight which is within the range of about 3,000 to about 50,000, whereinthe liquid isoprene-butadiene polymer has a glass transition temperaturewhich is within the range of about −50°C. to about 20°C., and whereinthe liquid isoprene-butadiene polymer exhibits only one glass transitiontemperature.
 2. A liquid isoprene-butadiene polymer as specified inclaim 1 wherein said liquid isoprene-butadiene polymer has a numberaverage molecular weight which is within the range of about 5,000 toabout 30,000.
 3. A liquid isoprene-butadiene polymer as specified inclaim 2 wherein said liquid isoprene-butadiene polymer has a glasstransition temperature which is within the range of about −40°C. toabout 10°C.
 4. A liquid isoprene-butadiene polymer as specified in claim2 wherein said liquid isoprene-butadiene polymer is comprised of repeatunits which are derived from about 20 weight percent to about 80 weightpercent isoprene and from about 20 weight percent to about 80 weightpercent 1,3-butadiene.
 5. A liquid isoprene-butadiene polymer asspecified in claim 4 wherein said liquid isoprene-butadiene polymer hasa number average molecular weight which is within the range of about8,000 to about 18,000.
 6. A liquid isoprene-butadiene polymer asspecified in claim 5 wherein said liquid isoprene-butadiene polymer hasa glass transition temperature which is within the range of about −30°C.to about 0°C.
 7. A liquid isoprene-butadiene polymer as specified inclaim 6 wherein said liquid isoprene-butadiene polymer is comprised ofrepeat units which are derived from about 30 weight percent to about 70weight percent isoprene and from about 30 weight percent to about 70weight percent 1,3-butadiene.
 8. A tire tread compound which iscomprised of (a) a rubbery elastomer, (b) about 20 phr to about 80 phrof an aromatic process oil, (c) about 35 phr to about 130 phr of afiller and (d) about 4 phr to about 40 phr of a liquidisoprene-butadiene rubber that is comprised of repeat units that arederived from about 5 weight percent to about 95 weight percent isopreneand from about 5 weight percent to about 95 weight percent1,3-butadiene, wherein the liquid isoprene-butadiene rubber has a numberaverage molecular weight which is within the range of about 3,000 toabout 50,000, wherein the liquid isoprene-butadiene rubber has a glasstransition temperature which is within the range of about −50°C. toabout 20°C., wherein the repeat units in the liquid isoprene-butadienerubber are in essentially random order, and wherein the liquidisoprene-butadiene polymer exhibits only one glass transitiontemperature.
 9. A tire tread compound as specified in claim 8 whereinthe liquid isoprene-butadiene rubber has a glass transition temperaturewhich is within the range of about 50°C. to about 20°C.
 10. A tire treadcompound as specified in claim 9 wherein the rubbery elastomer isstyrene-butadiene rubber.
 11. A tire tread compound as specified inclaim 10 wherein the liquid isoprene-butadiene rubber has a numberaverage molecular weight which is within the range of about 5,000 toabout 30,000; and wherein said liquid isoprene-butadiene rubber has aglass transition temperature which is within the range of about −40°C.to about 10°C.
 12. A tire tread compound as specified in claim 11wherein the liquid isoprene-butadiene rubber is present at a level whichis within the range of about 5 phr to about 25 phr.
 13. A tire treadcompound as specified in claim 12 wherein the oil is present at a levelwhich is within the range of about 20 phr to about 50 phr.
 14. A tiretread compound as specified in claim 12 wherein the oil is present at alevel which is within the range of about 30 phr to about 45 phr.
 15. Atire tread compound as specified in claim 12 wherein the oil is presentat a level which is within the range of about 50 phr to about 80 phr.16. A tire tread compound as specified in claim 15 which is furthercomprised of a resin.
 17. A tire tread compound as specified in claim 16wherein said resin is present at a level which is within the range ofabout 5 phr to about 60 phr.
 18. A tire tread compound as specified inclaim 17 wherein the filler is present at a level which is within therange of about 35 phr to about 70 phr.
 19. A tire tread compound asspecified in claim 18 wherein the liquid isoprene-butadiene rubber has anumber average molecular weight which is within the range of about 8,000to about 18,000; wherein said liquid isoprene-butadiene rubber has aglass transition temperature which is within the range of about −30°C.to about 0°C.; and wherein said liquid isoprene-butadiene rubber iscomprised of repeat units which are derived from about 20 weight percentto about 80 weight percent isoprene and from about 20 weight percent toabout 80 weight percent 1,3-butadiene.
 20. A tire which is comprised ofa generally toroidal-shaped carcass with an outer circumferential tread,two spaced beads, at least one ply extending from bead to bead andsidewalls extending radially from and connecting said tread to saidbeads; wherein said tread is adapted to be ground-contacting; whereinthe tread is comprised of (a) a rubbery elastomer, (b) about 20 phr toabout 80 phr of an aromatic process oil, (c) about 35 phr to about 130phr of a filler and (d) about 4 phr t rubber that is comprised of repeatunits that are derived from about 5 weight percent to about 95 weightpercent isoprene and from about 5 weight percent to about 95 weightpercent 1,3-butadiene, wherein the liquid isoprene-butadiene rubber hasa number average molecular weight which is within the range of about3,000 to about 50,000, wherein the liquid isoprene-butadiene rubber hasa glass transition temperature which is within the range of about 50°C.to about 20°C., wherein the repeat units in the liquidisoprene-butadiene rubber are in essentially random order, and whereinthe liquid isoprene-butadiene polymer exhibits only one glass transitiontemperature.